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Journal ArticleDOI

Consideration of SLM additive manufacturing supports on the stability of flexible structures in finish milling

01 Feb 2021-Journal of Manufacturing Processes (Elsevier)-Vol. 62, pp 213-220

Abstract: The supports in additive manufacturing can be used in an innovative way by being considered as supports for machining operation. This innovative use of manufacturing supports can facilitate the finishing of functional thin structures. But the flexible global workpiece-supports system can potentially cause vibrations during the machining operation. This can cause irregular surfaces with bad quality. This study highlights the importance of additively manufactured support structures on the stability of Ti-6Al-4 V parts milling by using supports as a machining fixture. Nowadays, the control of the support stiffness and mechanical properties is not proposed by specific AM software. A way to develop a numerical method to optimize the post-processing of additive manufacturing parts is to use specific lattice structures as supports. Indeed, by adjusting the topology and the beam diameter of lattices, the relative stiffness and the relative density of the global structure can be controlled. The objective of this study is to show that the stiffness of the manufacturing supports is crucial for the machining operation. To validate this concept, milling tests are proceeded on thin-walled plates produced by Selective Laser Melting (SLM) using defined finish milling cutting conditions. Three types of results are obtained: cutting forces signals, displacements and surface qualities by confocal microscopy. The study reveals that milling can induces chatters. Also, surface qualities and dimensional deviations depend on the support choice. The control of mechanical properties of support structures appears to be a good way to favor machining operation of flexible and thin-walled structures. Topology and dimensional parameters of supports have to be considered in preliminary design steps of the additive manufacturing digital chain.
Topics: Machining (65%), Selective laser melting (52%)

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Consideration of SLM additive manufacturing supports
on the stability of exible structures in nish milling
Paul Didier, Gael Le Coz, Guillaume Robin, Paul Lohmuller, Boris
Piotrowski, Abdelhadi Moufki, Pascal Laheurte
To cite this version:
Paul Didier, Gael Le Coz, Guillaume Robin, Paul Lohmuller, Boris Piotrowski, et al.. Considera-
tion of SLM additive manufacturing supports on the stability of exible structures in nish milling.
Journal of Manufacturing Processes, Society of Manufacturing Engineers, 2021, 62, pp.213-220.
�10.1016/j.jmapro.2020.12.027�. �hal-03096965�

Consideration of SLM additive manufacturing supports on the stability of flexible structures in finish
milling
P. Didier
1
, G. Le Coz*
1
, G. Robin
1
, P. Lohmuller
1
, B. Piotrowski
1
, A. Moufki
1
, P. Laheurte
1
1
Université de Lorraine, CNRS, Arts et Métiers ParisTech, LEM3, F-57000 Metz, France
Abstract:
The supports in additive manufacturing can be used in an innovative way by being considered as
supports for machining operation. This innovative use of manufacturing supports can facilitate the
finishing of functional thin structures. But the flexible global workpiece-supports system can
potentially cause vibrations during the machining operation. This can cause irregular surfaces with bad
quality.
This study highlights the importance of additively manufactured support structures on the stability of
Ti-6Al-4V parts milling by using supports as a machining fixture. Nowadays, the control of the support
stiffness and mechanical properties is not proposed by specific AM software. A way to develop a
numerical method to optimize the post-processing of additive manufacturing parts is to use specific
lattice structures as supports. Indeed, by adjusting the topology and the beam diameter of lattices, the
relative stiffness and the relative density of the global structure can be controlled. The objective of this
study is to show that the stiffness of the manufacturing supports is crucial for the machining operation.
To validate this concept, milling tests are proceeded on thin-walled plates produced by Selective Laser
Melting (SLM) using defined finish milling cutting conditions. Three types of results are obtained:
cutting forces signals, displacements and surface qualities by confocal microscopy. The study reveals
that milling can induces chatters. Also, surface qualities and dimensional deviations depend on the
support choice.
The control of mechanical properties of support structures appears to be a good way to favor
machining operation of flexible and thin-walled structures. Topology and dimensional parameters of
supports have to be considered in preliminary design steps of the additive manufacturing digital chain.
Keywords: Selective Laser Melting (SLM); Additive Manufacturing support; machining.
1. Introduction
The growing interest in Additive Manufacturing (AM) processes and particularly in the Selective Laser
Melting (SLM) technology resulted in the development of the associated techniques: “design for
additive manufacturing”. This offers new opportunities and considerations despite new constraints [1].
The possibilities offered of 3D printing technology make it possible to redesign products by taking
advantage of these new processes [2]. For example, in the biomedical field, it is possible to
manufacture custom-made implants and prosthesis which perfectly adapt to bone anatomy [3].
Automotive, aeronautical and aerospace industries are particularly attracted by the freedom of
shapes, allowing the manufacturing of topologically optimized macro-structures [4] and complex beam
network, such as lattices structures [5]. Numerical approaches of topological optimization allow to

determine the optimal distribution of material in a defined volume subjected to specific loads for the
lightening of the structure. [6]. All these new possibilities lead to complex geometries and thin-walled
structures.
Additive manufacturing technologies also impose various constraints [1, 7]. For instance, the process
to obtain powder for SLM technology is expensive and thus the achievable alloy compositions are
limited. The control of the process allows very low porosity rates but leads to anisotropic mechanical
properties and irregular surface roughness that are strongly dependent on the orientation of the parts
on the substrate [8, 9]. Residual stresses related to rapid cooling induce geometrical deviations [10].
During the manufacturing processes, manufacturing supports are always used to limit strains by
strengthening the overall structure and favoring a better heat diffusion during manufacturing [11].
However, support structures have to be removed afterward. This may take time and require manual
operations causing topological defects. These defects further aggravate roughness defects.
Although additive manufacturing process makes it possible to approach the near-net shape of the
product, finishing operations by chemical [12], mechanical [13] or laser treatment [14] are necessary.
For some surfaces, accurate dimensional qualities are required and thus finishing by milling or grinding
is necessary [15]. These treatments have already been integrated by some companies in their hybrid
machining center offering additive manufacturing and milling possibilities [16], where material
deposition and machining operation are performed in the same place. In the case of SLM process, the
hybrid solutions are more complicated due to the powder deposition. Thus, the complementarity
between the additive and the subtractive machines must be investigated and the repositioning
methods for machining of the produced part should be considered.
Figure 1 Machining operation of an additive manufactured part with consideration of supports as a machining fixture.
The concept investigated in this study is the following: considering the machining operation by milling,
it is possible to consider the supports no longer as a constraint, but as an opportunity, as a customized
machining fixture. The support structures are preserved in order to perform post-processing on
specific surfaces, see Figure 1. This approach allows a gain of operations to obtain finished functional
surfaces. This strategy eliminates the need to develop a specific clamp for a unique, custom-made,
complex-shaped part. Its potential leads to the cost-effectiveness of production where post-processing
is still considered to be artisanal. To achieve such a goal, several considerations need taking into
account. The first technical aspect that should be considered is the appropriate positioning and the
orientation of the printed part in order to make the considered surfaces accessible to the cutting tool.
The second is the geometry and the stiffness heterogeneity of the global manufacturing part, including

the manufacturing supports and the workpiece. These factors are unfavorable for the milling, due to
the bending of the flexible part, and thus vibrations can appear between the tool and the part [17].
This situation generates dimensional deviations and low surface quality [18]. It is then necessary to
understand the vibratory phenomenon in order to anticipate, control and limit it by optimizing cutting
speed and material removal rate [19-21]. Although the printed part is not adaptable because of the
function that it was designed for, the supports can be optimized. They can be used to modify the
mechanical properties of the part that supports the overall system, as sacrificial structures to increase
the stiffness [22] or a mass damper to adjust the eigenfrequency of the structure [23]. However, in the
digital chain dedicated to additive manufacturing, supports are principally designed to build an
unsupported overhang structure or to limit part distortion. Considering the literature review of Plocher
and Panesar [24], there is no previous work including the support structure optimization taking into
account all the mechanical loads applied during the post-processing. Indeed, the control of their
equivalent stiffness and their equivalent mechanical properties is not considered. In the same way as
Hussein et al. [11] proposes to use lattice structures as supports to minimize the lasering time. A novel
approach is to consider lattices structures as supports with the ability to control the mechanical
properties of the overall additive manufacturing part, with the objective of post-processing the
surfaces by milling.
Thus, the present work is proposed to use lattices structures as custom-made supports directly
manufactured during the process. This study highlights the influence of support stiffness on the milling
operation and its effect on the quality of the finished surfaces. Side milling tests are carried out on
plate samples and their manufacturing supports. Support structures with different stiffnesses are
compared, using different geometrical and sizes of support structures. Cutting forces and
displacements are measured using a dynamometric plate and a laser vibrometer. A correlation to the
surface quality is presented.
2. Material and methods
2.1. Geometry and SLM additive manufacturing of the samples
Differences of stability in milling and potential vibration problems are highlighted by the design of
adequate SLM samples. In the previous studies about chatters when machining, milling instability
directly resulted from the thin thickness of thin-walled plates. Thevenot et al. [19] milled a 1-millimeter
thin wall of steel plate (S235) and Seguy et al. [18] worked with a 2017 aluminum alloy workpiece with
3 mm in thickness and 20 mm in height.
In the present work, the system is composed of two parts of Ti-6A-l4V. The main deflection of the
plates is due to the open geometry of the supports and their low stiffness. Samples with two subparts
are considered and the geometry of the samples is presented in Figure 2. These two parts are
manufactured on a third block (unrepresented) that allows the clamping of the samples in the
machining center and ensures an embedment of the samples at the base of the support-plate system.
The upper rectangular-shaped plate, 3 mm thickness (l
1
), 9 mm high (l
2
) and 9 mm wide (l
4
), is the
subpart to be finished by machining. The lower part of the samples corresponds to the manufacturing
support. Its thickness is equal to (l
1
) and the height (l
3
) to 4.5 mm. As the geometrical shape is fixed,
the sample stiffness is controlled by the geometry and the architecture of the supports. Two families
of structures are considered in this study to cover a large range of stiffness: structures directly exported
from the SLM digital chain and lattices structures, as shown in Figure 2.

The first supports are obtained with the Magics® Materialise software, dedicated to the data
preparation for SLM. The Web support is composed of various vertical walls, one crossing another at
the center of the surface. The Block support consists of intersecting walls with diamond-shaped
perforations. The second type of supports proposed are lattice structures, made by the repetition of
an elementary unit-cell. By adjusting the topology and the beam diameter, the relative stiffness and
the relative density of the global structure can be controlled and numerically implemented. These
mechanical properties are essential to understand the dynamic phenomenon during milling. Indeed,
with the control of the truss diameter and the unit-cell size, and thus the control of the relative density,
it is possible to determine its Young’s modulus [25]. Two structures, well known in the literature, are
retained in this study: the Diagonal and the Octet-truss structure [26]. Two truss diameters are
considered for each cell: 0.300 and 0.375 mm. The same global unit-cell dimension of 1.5x1.5x1.5 mm
3
are used and the same pattern repetition (6x2x3) in the x, y and z directions are used to define the
global samples with fixed dimensions. For each structure, four samples are prepared, using the laser
manufacturing parameters for the Ti-6Al-4V alloy (Power P = 200 W, laser speed v = 1650 mm/sec and
hatching distance H = 80 µm).
Figure 2 - Geometry of the two types of sample: lattice structures and AM software structures
2.2. Analysis of the first modal frequency
The sample is composed of two parts: the upper plate and the lower support that has the equivalent
behavior of a bulk plate structure. Embedded in the base, the natural principal deformation is the
flexion around the y-axis, as shown in Figure 2. As all the samples have the same external dimensions,
they have the same first mode of vibration corresponding to the flexion around the y-axis. Since the
natural frequencies of the plate increase with its rigidity, the stiffness of each sample is analyzed
through the natural frequency of the first mode f
1
.
An impact hammer allows us to determine the resonance frequency of the structure. The
displacements are measured by a laser vibrometer. As there is no contact between the sensor and the
workpiece, the measurement is not disturbed. The frequency range of the solicitation is from 1Hz to
10 000Hz. The laser is focused in the middle of the free end of the plate. A Fast Fourier Transformation
(FFT) is performed to obtain the frequency content of the vibration signal. The first modal frequency
is scattered with a lower value of 743 Hz for the Diagonal with 0.3 mm diameter to 6656 Hz for the
Block sample, as given in Table 1. The dispersion is slightly increased for structures with 0.3 mm in
diameter (8.21 %) from that for the structures with 0.375 mm in diameter (4.59 %). Indeed, at this
scale, the geometrical defects due to the process are even more important with smaller diameters

Citations
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Pei Wang1, Si-Jie Yu1, Jaskarn Shergill, A. K. Chaubey  +7 moreInstitutions (6)
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1 citations


References
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Book
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Abstract: A new method for the analytical prediction of stability lobes in milling is presented. The stability model requires transfer functions of the structure at the cutter - workpiece contact zone, static cutting force coefficients, radial immersion and the number of teeth on the cutter. Time varying dynamic cutting force coefficients are approximated by their Fourier series components, and the chatter free axial depth of cuts and spindle speeds are calculated directly from the proposed set of linear analytic expressions without any digital iteration. Analytically predicted stability lobes are compared with the lobes generated by time domain and other numerical methods available in the literature.

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Abstract: The past few decades have seen substantial growth in Additive Manufacturing (AM) technologies. However, this growth has mainly been process-driven. The evolution of engineering design to take advantage of the possibilities afforded by AM and to manage the constraints associated with the technology has lagged behind. This paper presents the major opportunities, constraints, and economic considerations for Design for Additive Manufacturing. It explores issues related to design and redesign for direct and indirect AM production. It also highlights key industrial applications, outlines future challenges, and identifies promising directions for research and the exploitation of AM's full potential in industry.

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Abstract: Recent research on the additive manufacturing (AM) of Ti alloys has shown that the mechanical properties of the parts are affected by the characteristic microstructure that originates from the AM process. To understand the effect of the microstructure on the tensile properties, selective laser melted (SLM) Ti–6Al–4V samples built in three different orientations were tensile tested. The investigated samples were near fully dense, in two distinct conditions, as-built and stress relieved. It was found that the build orientation affects the tensile properties, and in particular the ductility of the samples. The mechanical anisotropy of the parts was discussed in relation to the crystallographic texture, phase composition and the predominant fracture mechanisms. Fractography and electron backscatter diffraction (EBSD) results indicate that the predominant fracture mechanism is intergranular fracture present along the grain boundaries and thus provide and explain the typical fracture surface features observed in fracture AM Ti–6Al–4V.

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